1. Field of the Invention
The present invention relates to an etching solution and more particularly, to an etching solution for a copper (Cu) single metal layer or copper (Cu)/titanium (Ti) double metal layer.
2. Discussion of the Related Art
FIG. 1 is a perspective view of a liquid crystal display (LCD) device according to the related art. In FIG. 1, a liquid crystal display (LCD) device has upper and lower substrate 5 and 10, and a liquid crystal material layer 9 interposed between the upper and lower substrate 5 and 10. The upper substrate 5 has color filters 7, black matrices 6 between each color filters 7, and a common electrode 18 that is formed on the color filters 7 and the black matrices 6. The lower substrate 10 has a pixel region “P,” a pixel electrode 17 formed within the pixel region “P,” a plurality of switching elements “T” and array lines. The lower substrate 10 is commonly referred to as an array substrate and a plurality of the switching elements “T” are formed at cross points of gate and data lines 14 and 22. The switching elements include thin film transistors “T” arranged in a matrix form. The pixel region “P” is defined by the gate and data lines 14 and 22 that cross each other and the transparent pixel electrode 17 is formed within the pixel region “P”. The pixel and common electrodes 17 and 18 are formed of transparent conductive material, such as indium tin oxide (ITO), that is relatively superior in transmission of light. A liquid crystal display (LCD) device commonly uses optical anisotropy and polarization properties of liquid crystal molecules. The thin film transistor “T” has a gate electrode, a source electrode, a drain electrode, and a semiconductor layer. The gate electrode needs to be formed of material that has a low electric resistance and may include one of a copper (Cu) single layer and a copper (Cu)/titanium (Ti) double metal layer.
FIGS. 2A to 2D are cross sectional views of a fabricating sequence of an array substrate of a liquid crystal display (LCD) device according to the related art. In FIG. 2A, a gate line 14 (in FIG. 1) and a gate electrode 30 are formed by depositing a copper (Cu)/titanium (Ti) double metal layer on a transparent glass substrate 10, and then patterning the double metal layer. A gate insulating layer 32 is formed on the gate electrode 30 by depositing one of inorganic insulating materials, such as silicon nitride (SiNx) and silicon oxide (SiO2), on the substrate 10. An active layer 34 and an ohmic contact layer 36, which are laminated over the gate electrode 30, are formed on the gate insulating layer 32. The active layer 34 is formed of pure amorphous silicon, and the ohmic contact layer 36 is formed of impurity-doped amorphous silicon.
In FIG. 2B, source and drain electrodes 38 and 40 and a data line 22 are formed by depositing the copper (Cu)/titanium (Ti) double metal layer on the substrate 10, and then patterning the double metal layer. The source electrode 38 contacts the ohmic contact layer 36, and the drain electrode 40 is spaced apart from the source electrode 38. The data line 22 is connected to the source electrode 38.
In FIG. 2C, a passivation layer 42 is formed by coating one of inorganic insulating materials, such as silicon nitride (SiNx) and silicon oxide (SiO2), or transparent organic insulating materials, such as benzocyclobutene (BCB) and acrylic resins, on the gate insulating layer 32 over the substrate 10. A drain contact hole 46, which exposes a portion of the drain electrode 40, is formed through the passivation layer 42 by patterning the passivation layer 42.
In FIG. 2D, the transparent pixel electrode 17, which contacts the drain electrode 40 through the drain contact hole 46, is formed on the passivation layer 42.
A copper (Cu) single metal layer or a copper (Cu)/titanium (Ti) double metal layer for forming the gate electrode 30 and the source and drain electrodes 38 and 40 is patterned by a wet etching method using an etching solution. Oxone (2KHSO5.KHSO4.K2SO4) is commonly used as the etching solution for the copper (Cu) single metal layer and a mixture of the oxone, hydrofluoric acid (HF), and ammonium fluoride (NH4 F) is commonly used as the etching solution for the copper (Cu)/titanium (Ti) double metal layer.
A copper (Cu) etching mechanism by the oxone is as follows:KHSO5→K++HSO5  (1)HSO5−→H++SO52−  (2) Cu+SO5−→CuO+KHSO4  (3)Cu+KHSO5→CuO+KHSO4  (4)CuO+2KHSO4→CuSO4+K2SO4+H2O  (5)
However, in case of KHSO4, it undergoes reactions as follows:KHSO4→K++HSO4−  (6)HSO4−+H2O→HSO5−+2H++2e−  (7)
The HSO5− ion is resolved as in equation (8) when a hydrogen ion concentration (pH) is high or transition ions serving as a catalyst exists, and thus etching rate is accelerated.HSO5−→H++SO52−  (8)
The etching rate is accelerated until a concentration of KHSO4 is reduced, and then the etching rate starts to be decelerated when the concentration of KHSO4 passes the peak. In the previously stated reaction formulas, a reactant that is actually used for the reaction among the mixture of the oxone is KHSO5. KHSO5 is very unstable when it solely exists and thus is easy to be dissolved. Accordingly, it exists in a form of a mixture as in the mixture of oxone (2KHSO5.KHSO4.K2SO4). The K2SO4 is produced as a product after the reaction as in the equation (5). An initial etching rate using the oxone as an etching solution is increased as a catalytic reaction is activated by an increase of copper (Cu) ions. That is, the copper (Cu) ions that are produced in the following reaction (9) and (10) facilitate an etching of the copper (Cu) metal layer.
 Cu+2+2e→2Cu+2+2e  (9)Cu→2Cu+2+2e  (10)
Accordingly, the copper (Cu) metal layer is etched as in the following reaction:2Cu+2+Cu→2Cu+1+Cu+2
FIG. 3 is a graph illustrating a change of an etching rate as a copper concentration of etching solution increases according to an increase of a number of times the etching solution is used according to the related art. In FIG. 3, the etching rate is increased as a concentration of the copper (Cu) is increased, and the etching rate is decreased as a concentration of the KHSO4 is lowered due to consumption. That is, during an increasing section of the etching rate, the etching rate is increased as the copper (Cu) ions are increased and subsequently activating a catalytic reaction which increases a concentration of SO52−. During a decreasing section of the etching rate, the etching rate is decreased as KHSO5.KHSO4 is consumed. Because the etching rate in not uniform due to a changing amount of the copper (Cu) ion, it is difficult for a side of a patterned copper (Cu) metal layer to obtain a taper angle. In addition, when the mixture of the oxone and hydrofluoric acid (HF) (or ammonium fluoride (NH4 F)) is used as an etching solution for etching the copper (Cu)/titanium (Ti) double metal layer, it is possible for both sides of the patterned metal layer to obtain a taper angle. However, the taper angle is reduced owing to a difference between the etching rate of the copper (Cu) by the oxone and the etching rate of the titanium (Ti) by hydrofluoric acid (HF) and ammonium fluoride (NH4 F) and owing to a lateral etching by the oxone, and thus a critical dimension loss (CD loss) is caused.
FIG. 4 is an electron microscopic photomicrograph of a cross-section of a photoresist and a copper (Cu)/titanium (Ti) double metal layer after a wet-etching process according to the related art. In FIG. 4, region “A” is a glass substrate, region “B” is a patterned copper (Cu)/titanium (Ti) double metal layer on the glass substrate, and region “C” is a photoresist on the copper (Cu)/titanium (Ti) double metal layer. A side “D” of the copper (Cu)/titanium (Ti) double metal layer is over etched when the etching solution is used for etching the copper (Cu)/titanium (Ti) double metal layer. Moreover, hydrofluoric acid (HF) and ammonium fluoride (NH4 F) that are used for etching the titanium (Ti) metal layer generate damage on a surface of the glass substrate when an amount of the hydrofluoric acid (HF) and ammonium fluoride (NH4 F) is excessive in the etching solution.
FIG. 5 is an electron microscopic photomicrograph illustrating damage on a surface of a glass substrate after a wet etching process in which hydrofluoric acid (HF) is over used according to the related art. In FIG. 5, if the hydrofluoric acid (HF) or the ammonium fluoride (NH4 F) in the mixture of the oxone and the hydrofluoric acid (HF) (or ammonium fluoride (NH4 F)) is over used, the surface “B” of the glass substrate is seriously damaged. If there occurs damage on the surface of the glass substrate occurs, the critical dimension loss (CD loss) may occur between the glass substrate and a layer structure that will be formed during a later process. To prevent these problems, the amount of the hydrofluoric acid (HF) or the ammonium fluoride (NH4 F) may be reduced. However, if the amount of the hydrofluoric acid (HF) or the ammonium fluoride (NH4 F) is reduced, the titanium (Ti) metal layer will not be completely removed, and a residue of the titanium (Ti) metal layer will remain.